The halogenated dibenzo-p-dioxins and dibenzofurans are environmental contaminants which are formed during the commercial synthesis of certain o-chlorophenol products. One of these, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) is perhaps the most potent small molecule toxin and teratogen known. We are studying the chemistry and biological actions of TCDD and analogues. TCDD was found to be a potent inducer of ALA synthetase and aryl hydrocarbon hydroxylase. The potency of dioxin congeners as toxins exactly corresponds to their potency at inducing these two enzymes. We are pursuing these studies: 1) to further synthesize analogues of TCDD and examine their toxic potency and inducing potential; 2) to explain the mechanism by which TCDD exerts its toxic action; 3) to use these compounds to understand the genetic expression of aryl hydrocarbon hydroxylase.